USGS Scientists Document Widespread Increases in Streamflow and
Changes in the Timing of Snowmelt Over the Past 50 Years
Reston, VA  U.S. Geological Survey (USGS) scientists have identified
nationwide trends toward increasing streamflow in many areas of the
nation since 1940 based on data collected from long-term USGS
streamgages. This conclusion and several more interesting trends in
our nation's streamflows can be found in four new fact sheets
recently issued by the agency.

"Understanding streamflow trends is essential to effective management
of the nation's water supply and is critical to developing strategies
that mitigate the potential negative impacts of floods and droughts,"
said USGS Associate Director for Water Robert Hirsch.

In the first study, USGS scientists identified a nationwide trend
that streamflow has been increasing in the United States since at
least 1940. Most of the increases were during low-and-moderate
streamflows. This means that, during typically dry periods, more
water is now available in the stream.

In the second study, scientists discovered that over the last 30
years, winter/spring streamflows occurred one to two weeks earlier
than in previous decades in northern or mountainous areas of New
England. Similarly, in the third study, scientists found that
streamflows in most western rivers occur almost one to three weeks
earlier now than they did in the middle of the 20th century.

The fourth study shows that the streamflow of the Mississippi River
was influenced by both climate and human activities such as
construction of water reservoirs, agricultural irrigation and
groundwater pumping. Streamflow of the Mississippi River increased at
a rate of 4.5 percent per decade largely because of an increase in
precipitation.

The USGS has been measuring and recording streamflow in the United
States since the late 1800's. Today, the USGS monitors streamflow at
7,400 locations nationwide. The USGS streamflow information is used
for many purposes such as water resource appraisal and allocation,
design of the nation's infrastructure such as bridges and water
treatment plants, flood hazard planning, National Weather Service
flood forecasting, reservoir operations, water-quality management,
habitat assessment and protection, recreational enjoyment and safety,
and understanding changes in streamflow due to land-use and climate
changes. USGS streamflow data are available at http://water.usgs.gov/waterwatch/.

Summary of the Fact Sheets:

Streamflow Trends in the United States
(http://pubs.water.usgs.gov/fs2005-3017/)
Streamflow has been increasing in the United States since at least
1940. Regions that experienced the most widespread increases were the
Upper Mississippi, Ohio Valley, Texas-Gulf, and the Mid Atlantic.
· Of the nearly 22,700 streamgages for which the USGS has records,
435 monitor natural basins and have records of sufficient length to
analyze climatic trends.
· Streamflow increased across most of the United States during the
20th century at 40-45 percent of these 435 stations.
· Increases were most prevalent in low to moderate streamflows (seen
at 40 percent of the stations), with relatively few decreases (seen
at 8 percent of stations).
· Comparatively few stations (10 percent) had increases in annual
maximum streamflow.
· Streamflow increases occurred as a sudden rather than gradual
change around 1970, suggesting the climate shifted to a new regime.

During the last 30 years, the timing of winter/spring streamflow has
shifted earlier by one to two weeks in northern and mountainous New
England streams.
· The date when half of the total volume of streamflow for
winter/spring (January 1 to May 31) now arrives earlier than it did
in the first half of the 20th century at 14 of 27 streamgages in New
England.
· This shift to earlier streamflow was evident at all of the gages in
the northern and mountainous areas of Maine and New Hampshire where
snowmelt has the greatest effect on streamflow (11 of the 27
streamgages).
· Only 4 of the 27 streamgages exhibited shifts in the timing of
fall/winter streamflow (October 1 to December 31), and all of these
tended toward earlier streamflow.

As much as three-quarters of water supplies in the western United
States are derived from snowmelt. Trends toward earlier snowmelt and
streamflow need to be considered in the water-resource and flood-
management systems and procedures in many western settings.
· The average streamflow center-of-volume date (the date on which one-
half of the total annual flow volume passes a streamgage) in the
western United States is about nine days earlier now than in the
1950s.
· These shifts in timing result both from late winter and early
spring temperature increases, and from changes in the form of
precipitation (increasing liquid precipitation, smaller percentage of
snow) in late winter and early spring.

This study involved analysis of trends in precipitation, streamflow,
evapotranspiration, depletion of ground water, and the filling of
reservoirs. This study describes the influences of both climate
trends and human alterations on streamflow from 1949 to 1997.
· Streamflow in the Mississippi River basin increased at a rate of
4.5 percent per decade during the second half of the 20th century.
· This increase resulted primarily from an increase in precipitation
offset by increases in evaporation from reservoirs and irrigated
cropland in the basin.

The USGS serves the nation by providing reliable scientific
information to describe and understand the Earth; minimize loss of
life and property from natural disasters; manage water, biological,
energy, and mineral resources; and enhance and protect our quality of
life. Subscribe to receive the latest USGS news releases.

* * *www.usgs.gov* * *

Your message has been successfully submitted and would be delivered to recipients shortly.